1. Field of the Invention
The present invention relates to a human-interactive type robot system where a robot and a worker simultaneously share the same work space to perform interactive work.
2. Description of the Related Art
General industrial robots perform work in spaces which are surrounded by safety fences so as to ensure the safety of workers. In recent years, demand has been increasing for human-interactive type robot systems which perform work sharing the same work space as workers.
In work which uses a human-interactive type robot, sometimes the robot and a worker perform separate work in a shared work space and sometimes a worker performs work on a workpiece which is held by the robot. In such a human-interactive type robot, since the worker and robot share the work space, the robot might contact the worker and injure the worker.
For this reason, the contact force between the robot and a worker is detected by a force sensor. When the detected contact force exceeds a predetermined threshold value, the robot is made to stop or the robot is made to operate so that the contact force becomes smaller to thereby ensure the safety of the worker. To reduce the danger to a worker, the threshold value of the contact force is preferably set to as small a value as possible.
For example, Japanese Patent Publication No. 8-39467A discloses a method of using a force sensor which is set at an end effecter of an industrial robot to detect a contact force to detect whether the end effecter has contacted a workpiece. Further, Japanese Patent No. 4643619 discloses that a change of temperature or aging of the force sensor causes a signal of the force sensor to change and discloses to correct the resultant drift which is included in the force detection value. Furthermore, Japanese Patent Publication No. 2007-30496 discloses to correct the drift which is included in the output from a strain detection device which was provided in a clamping device of a molding machine and to thereby detect the clamping force with a higher precision.
In this regard, a force sensor often includes a strain gauge. It uses the strain gauge to detect the amount of strain, converts the amount to force, and detects the direction and magnitude of the acting load. A strain gauge is generally attached by an adhesive to part of the body of the force sensor.
However, it is known that even if the same magnitude of force acts, the creep accompanying aging of a binder layer which is formed by an adhesive, a change of temperature, a change of humidity, etc. sometimes cause the amount of strain converted to value of force to include considerable drift from the actual value. In such a case, if the amount of strain which is additionally generated due to the above factors becomes larger, the drift also becomes larger. For this reason, the situation can arise where the force sensor will not recognize a contact force unless a certain degree of large contact force acts. If in this way the contact force which the force sensor can recognize becomes larger, there is a greater possibility that a worker will be endangered. Therefore, it is desirable that as small as possible a contact force can be detected with a high precision. Note that, sometimes even with other designs of force sensors, the detection value will similarly change along with the elapse of time.
The present invention was made in consideration of such a situation and has as its object the provision of a human-interactive type robot system which can detect a contact force by a high precision at all times even if the contact force by which a robot contacts a worker is small.